The entire disclosure of Japanese Patent Application No. 2000-203529 filed on Jul. 5, 2000 including specification, claims, drawings and summary is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to an apparatus for producing reduced iron by reducing pellets or briquette-like agglomerates, formed from a powdery mixture of an iron oxide powder and a reducing agent, in a high temperature atmosphere.
2. Description of the Related Art
To produce reduced iron, the first step is, generally, to mix a powder of iron ore (iron oxide), a powder of coal (reducing agent), a powder of limestone (fluxing agent), and a binder such as bentonite, and to compress and pelletize the mixture to form wet balls called green balls. Then, the wet balls are dried to some degree to form dry balls. The dry balls are heated to a high temperature in a reducing furnace, where iron oxide in the iron ore is reduced with the coal as a reducing agent to form reduced iron in the form of pellets.
An example of a conventional apparatus for producing reduced iron is explained by way of FIG. 6. Powders of iron ore, coal, etc. and a binder are mixed in a mixer (not shown). The resulting mixed powder is pelletized in a pelletizer 001 to form green balls (raw pellets) GB. Then, the green balls GB are charged into a dryer 002, where they are dried with an off-gas from a reducing furnace 004 (to be described later on) to form dry balls DB. The dry balls DB are supplied to the reducing furnace 004 by a pellet feeder 003.
The interior of the reducing furnace 004 is maintained in a high temperature atmosphere upon heating by a burner 005, and an inside off-gas is discharged from an off-gas duct 006. Thus, the dry balls DB are preheated and heated with radiant heat from the wall of the furnace when they are passed through the interior of the reducing furnace 004. During their passage, the iron oxide in the iron ore is reduced with the coal as the reducing agent to form reduced iron in the form of pellets. The reduced pellets are discharged into a pellet discharger 008, and accommodated into a portable vessel 009.
The off-gas from the off-gas duct 006 usually contains some unburned gas, and is thus burned in an after burner chamber 007 nearly completely. Then, the off-gas is cooled in a water spray primary cooler 010, and then sent to a heat exchanger 011, where it undergoes heat exchange to heat combustion air. Combustion air heated by the heat exchange is sent to the reducing furnace 004, and fed into the furnace together with a fuel. On the other hand, the off-gas is cooled again in a secondary cooler 012, and part of it is sent to the dryer 002 as drying air for the green balls GB as stated earlier. All of the off-gas is then cleaned in a dust collector 013, and released into the atmosphere via a stack 014.
In the conventional apparatus for producing reduced iron, as described above, heat exchange is performed by the heat exchanger 011 between the off-gas discharged from the reducing furnace 004 and combustion air. The heated combustion air is supplied to the reducing furnace 004, where the dry balls DB are preheated and heated with radiant heat from the furnace wall. The temperature of the off-gas may be as high as about 1,300xc2x0 C., so that the off-gas has a great amount of thermal energy. Conversely, the metallic recuperative heat exchanger 011 is thermally resistant to temperatures of about 900xc2x0 C. or lower because of its structure. Thus, the off-gas is cooled by the water spray primary cooler 010 before it is sent to the heat exchanger 011. The dryer 002 for the green balls GB has a structure designed only to perform drying of the green balls GB. To prevent rupture of the pellets, the gas for drying also needs to be cooled to about 300xc2x0 C. or lower (desirably about 270xc2x0 C.). To adjust the temperature of the off-gas from the recuperative heat exchanger 011, the water spray secondary cooler 012 is provided to add water into the off-gas and lower the temperature of the gas to be supplied to the dryer 002, by utilizing the heat of vaporization of water.
As described above, the secondary cooler 012 is also needed in addition to the water spray primary cooler 010, so that the system for treatment of the off-gas is complicated. Besides, the amount of the off-gas increases at least by the amount of the water sprays used. Thus, the treatment system for the off-gas is upsized. Moreover, the dry balls DB are preheated by the radiant heat with low thermal efficiency in the reducing furnace 004, and the latent heat of evaporation of the off-gas is taken away by the water spray. That is, much of the heat in the off-gas is wasted. For such reasons, recovery of the sensible heat possessed by the off-gas (i.e., effective use of the sensible heat) is insufficient. Hence, consumption of fuel used in the reducing furnace 004 is increased thereby raising the fuel cost, and the equipment (reducing furnace) is upsized.
The present invention has been proposed in light of these circumstances. It is an object of this invention to provide an apparatus for producing reduced iron, which can decrease the fuel cost and downsize the equipment by effective use of the sensible heat of the off-gas discharged from the reducing means, and which can downsize and simplify a system for treatment of the off-gas by decreasing the amount of the off-gas.
According to the present invention, as a means of attaining the above object, there is provided an apparatus for, producing reduced iron by drying agglomerates, which are pelletized from a powdery mixture of an iron oxide powder and a reducing agent, by a drying means; preheating the dried agglomerates by a preheating means; and then reducing the preheated agglomerates in a high temperature atmosphere of a reducing means, wherein the preheating means convects an off-gas from the reducing means to preheat the dried agglomerates. Thus, a decrease in the fuel cost, and a downsizing of the equipment can be achieved by the effective use of the sensible heat carried by the off-gas discharged from the reducing means. Moreover, a downsized, simplified system for treatment of the off-gas is realized by decreasing the amount of the off-gas.
In the apparatus for producing reduced iron, the drying means and the preheating means may be integrally formed as a drying preheater for drying and preheating a continuous flow of the agglomerates. Thus, the reducing means can be downsized, and the drying preheater can be made more compactly.
In the apparatus for producing reduced iron, a combustion means may be provided for burning any unburned gas in the merged off-gas, and part of the off-gas from the combustion means may be cooled with air and supplied to the drying means to dry the agglomerates. Thus, the unburned gas in the off-gas flowing in the off-gas circulation loop can be completely burned, and the temperature of the off-gas fed to the drying means can be lowered effectively.
In the apparatus for producing reduced iron, any unburned gas contained in the part of the off-gas discharged from the preheating means may be burned using part of the combustion air which is supplied to the reducing means, and then the part of the off-gas may be supplied to the drying means. Thus, the unburned gas can be burned effectively, and this is useful when dry distilled coal or the like is used as the reducing agent in the raw pellets.
In the apparatus for producing reduced iron, a regenerative heat exchanger may be provided for heating combustion air to be supplied to the reducing means. Thus, the amount of the off-gas and the fuel for heating of the reducing means can be further decreased overall.